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photons has no mass ,in relativity any thing that moves in the speed of light has the mass of infinite ,then photons have to be with the mass of infinite, so why does photons are being an exceptional case ?

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    $\begingroup$ Possible duplicate of Why can't photons have a mass? $\endgroup$ – John Rennie Aug 18 '16 at 16:27
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    $\begingroup$ Possible duplicate of How can a photon have no mass and still travel at the speed of light? $\endgroup$ – Bosoneando Aug 18 '16 at 16:28
  • $\begingroup$ Please read the duplicates listed above, but to directly answer your question, a photon is not exceptional in being massless, the (hypothetical) graviton is conjectured to be massless and the gluon, carrier of the strong force, is also considered massless. However, unlike photons, gluons are never observed as free particles, since they are confined within hadrons. en.wikipedia.org/wiki/Massless_particle $\endgroup$ – user108787 Aug 18 '16 at 16:45
  • $\begingroup$ It is not the case that anything that moves at the speed of light has infinite mass. It is the case that anything that moves at the speed of light has a mass that is an infinite multiple of its mass when it is at rest. Therefore particles that have zero rest mass could have finite mass when travelling at the speed of light. $\endgroup$ – Martin Kochanski Aug 18 '16 at 17:08
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Photon doesn't have any rest mass. According to Einstein, anything which have rest mass would have infinite mass at speed of light. m' = mγ. As velocity tends to c, the factor γ tends to infinity. Thus m would be infinite. But for photons, at speed of light, m = 0/0 (actually meaningless!!). But would mean it is a finite mass.

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